Laser cladding by powder injection is one method for advanced material processing, which is used in manufacturing, part repairing, metallic rapid prototyping, and coating. A laser beam melts powder and a thin layer of the substrate to create a coat on the substrate. In this process, a great variety of materials can be deposited on a substrate to form a layer with a thickness of 0.1 to 2 mm. This technique can produce much better coating, with minimal dilution, minimal distortion, and good surface quality than other techniques such as arc welding and thermal plasma. These advantages have recently found attraction in industries for part manufacturing and metallic rapid prototyping, see J. Mazumder, D. Dutta, N. Kikuchi, A. Ghosh, “Closed loop direct metal deposition: art to part,” Optics and Lasers in Engineering, Vol. 34, pp. 397–414, 2001. This method is also considered as the best way for manufacturing of smart parts and functionally graded parts, see L. Xue, A. Therialult, M. U. Islam, “Laser consolidation for the manufacturing of complex flex tensional transducer shells,” Proceeding of ICALEO′2001, pp. 702–711, 2001.
In part manufacturing and metallic rapid prototyping using the laser cladding, similar to other rapid prototyping techniques, a three-dimensional CAD solid model is used to produce a part without intermediate steps. This approach to produce a mechanical component in a layer-by-layer fashion allows us to fabricate a part with features that may be unique to laser cladding prototyping. These features include homogeneous structure, enhanced mechanical properties, and complex geometry. However, the clad quality may vary significantly during a laser cladding process. Variations of the quality may even be observed between processing cycles performed by the same operating conditions. This poor reproducibility arises from the high sensitivity of laser cladding to small changes in the operating parameters such as laser power, beam velocity and powder feed rate, as well as to process disturbances such as variations in absorptivity. Finding an optimal set of parameters experimentally, and using them in an open loop laser cladding process may not result in a good quality clad due to disturbances in the system. As a result, a closed loop control system is essential for automating laser cladding process.
Some researchers have developed methods and systems for the control, improvement and monitor of laser cladding process. Their works fold into three categories: sensors for monitoring the process, closed loop control system, and especial devices for the process such as nozzles, powder feeder, optics, and motion systems.
Work on sensor development for real-time monitoring of laser material processing has been underway for several years. U.S. Pat. No. 5,659,426 and conference paper G. Kinsman, W. W. Duley, “Fuzzy logic control of CO2 laser welding,” Proceeding of ICALEO′1993, pp. 160–167, 1993, discloses how a vision system can be used to monitor the process zone for a laser material processing.
However, to date the use of vision systems based on CCD technology in which the output of vision system is used in a closed loop control system has been limited to the extraction of spatial information on the boundary of the interaction zone (melt pool) and determination of the number of bright pixels in the images (U.S. Pat. No. 5,659,479) or limited to the determination of clad height in an open-loop system (R. F. Meriaudeau, F. Truchetet “Image processing applied to laser cladding process,” Proceeding of ICALEO′1 996, pp. 93–103. 1996). U.S. Pat. No. 6,122,564, and journal paper J. Mazumder, D. Dutta, N. Kikuchi, A. Ghosh, “Closed loop direct metal deposition: art to part”, Optics and Lasers in Engineering, Vol. 34, pp. 397–414, 2001; and conference paper J. Choi, and Y. Hau, “Adaptive laser aided DMD process control,” Proceeding of ICALEO′2001, pp. 730–738, 2001; disclose the use of a phototransistor for process monitoring of a laser cladding process with maximum 20 Hz. The use of a phototransistor in a closed loop system has been limited to the taking of clad height deviation in a desired threshold. This method provides the maximum 0.25 mm precision in produced parts.
Controller development for closed loop control systems has been carried out by several researchers. U.S. Pat. No. 5,659,426 and conference paper G. Kinsman, W. W. Duley, “Fuzzy logic control of CO2 laser welding,” Proceeding of ICALEO′1993, pp. 160–167, 1993, disclose a fuzzy logic controller for manipulating the laser processing variables such as laser power, laser intensity and laser beam velocity to control the penetration depth welding of material. International PCT Patent application 00/00921, journal paper J. Mazumder, D. Dutta, N. Kikuchi, A. Ghosh, “Closed loop direct metal deposition: art to part,” Optics and Lasers in Engineering, Vol. 34, pp. 397–414, 2001 and conference paper J. Y. Jeng, S. C. Peng, C. J. Chou, “Metal rapid prototype fabrication using selective laser cladding technology,” International Journal of Advanced Manufacturing Technology, Vol. 16, pp. 681–687, 2000; disclose a feedback controller for adjusting the laser power based on the presence or absence of the laser beam from the deposit. This controller trims the control analogue voltage, which is sent to the laser based on the TTL signal received from phototransistor. The modified analogue signal sent to laser causes the laser beam's shutter to be on and off for specific durations.
A few inventors have introduced new devices for the laser cladding process. U.S. Pat. Nos. 6,269,540; 6,268,584 and 6,046,426 disclose devices which can be used in parts production such as turbine blades, valves and so on. The devices present the use of different form of spray nozzles such as coaxial, and lateral. They also disclose the use of four separated laser beams where the spray nozzle is located in the middle of them, or the different orientation of laser beam relative to the workpiece and spray nozzle.
It would be advantageous to provide a method and apparatus to extract the clad characteristics such as dimensions and metallurgical qualities in real-time with high precision.